The purpose of this study is to explore the genetics of hereditary prostate cancer (HPC). The Finnish population is uniquely suited for studies of the genetics of prostate cancer for a number of reasons. The population is relatively homogeneous because most of its members trace back to a small number of settlers. This situation greatly increases the likelihood that a gene or genes predisposing to HPC can be identified in such a population. Furthermore, Finland has developed comprehensive and complete central databases on the medical characteristics of its population. These databases facilitate identifying individuals and families with various medical conditions such as prostate cancer, saving time compared to doing the same study in a country like the United States, where records are dispersed and very incomplete or nonexistent. With the new technologies developed by the Human Genome Project at hand, the time is ripe for addressing the question of the role of genes in HPC and the Finnish population provides the ideal environment in which to find the best concentration of affected families. To facilitate the identification of the HPC gene, Finnish families with multiple prostate cancer cases will be identified. Finnish patients are likely to share a common ancestral mutation predisposing them to HPC. The linkage disequilibrium phenomenon will be used to pinpoint the location of this gene in the human genome. The Finnish population will be a model for the development of technologies for addressing the difficulties associated with the analyzing genetics of common diseases.